The invention relates to the production of a molded article from glass or glass ceramic.
It is known that in the shaping of a molded article from glass, in a certain temperature region or viscosity region the glass becomes sticky. With this, the glass adheres firmly to the mold. The critical viscosity lies at approximately 108 dPas.
EP 070 760 A1 shows and describes a method as well as a device for producing a molded article from glass. Here, a lump of the hot, molten mass is transferred into a mold. The walls of the mold are porous. The pores are linked by gas connections so as to be conductive. If gas is introduced into the connections, then this gas emerges from the pores and prevents contact between the glass and the inner surfaces of the mold walls.
The mold just mentioned, a so-called levitation mold, has a temperature that, in general, lies below the critical adhesion temperature of the glass. In this case, by virtue of the heat content in the glass lump, a certain temperature difference between the levitation mold and the glass lump can be tolerated.
It is not always desired to transfer the hot molten mass immediately into a levitation mold. One often wishes, instead of this, to first produce preforms from glass, and to then heat these up in a levitation mold, in order to give them there a final shape. This means that the preform must be heated in the levitation mold itself. See, for example, JP 61/14146. The preform is thus brought in a cold state into the levitation mold. There, it must be heated to a shaping temperature, which requires considerable amounts of heat and relatively long time periods. During the heating up, the temperature of the preform passes through the critical zone. In this, there exists the danger of an adhering of the glass to the walls of the mold container, regardless of the fact that the preform hovers due to the gas.
The invention is based on the task of bringing a preform, which is present in a cold state, to a shaping temperature and forming it in a shaping machine without the danger of an adhesion.
This task is accomplished through the features of the independent claims.
In this, the inventors have reached a manner of solution that is equally simple and correct. Accordingly, they propose, first of all, to produce the preform in the conventional manner, to transfer it into a levitation mold, to preheat it in the levitation mold to a temperature that lies below the adhesion temperature of the glass, then to apply a heat impulse to the preform, by means of microwave action with simultaneous gas feed, so that the preform is in a state of hovering, and finallyxe2x80x94after exceeding the critical adhesion temperature and reaching the desired shaping temperaturexe2x80x94to transfer the preform into a shaping machine.
The heating up of the preform thus takes place in two phases. In the first heating phase, the preform is heated to a temperature that lies below the critical adhesion temperature. In the second heating phase, the adhesion temperature is exceeded and the higher temperature necessary for the shaping is reached.
The invention is associated with the following advantages:
The first heating phase of the preform causes no kind of technical problems. The manner and duration of the first heating phase are unimportant and thus unproblematic. During the first heating phase no gas levitation is needed.
The second heating phase, by virtue of the choice of the heating means (namely, a microwave resonator), takes place extremely quickly. This second heating phase requires only a few seconds. Thus, no long holding times on the gas levitation membrane are necessary. Possible contacts between the preform and wall of the mold are so minimal, if present at all, that adhesions do not occur or only to the least degree.
The amount of gas necessary for the levitation is small, due to the short time span of the second heating phase.
The energy loss consequent of the transfer of heat from the now very hot preform to the relatively cool surrounding (membrane) is likewise small, due to the short time span of the second heating phase.
In contrast to the prior art, only the preform is heated, but not the surrounding, and thus also the membrane of the mold, so that a further energy saving results, in comparison to conventional methods and devices.
The levitation mold can be kept below the adhesion temperature, so that short-term contacts between mold and glass are also uncritical.
The two elements according to the inventionxe2x80x94levitation on the one hand, and application of the microwave heating on the other handxe2x80x94are decisively important in their combination. If one were to carry out the heating up only with microwave heating on a conventional mold (without levitation), then the adhesion problem would arise, since in this case the mold assumes, by means of heat conduction, the temperature of the product to be heated up. Since the adhesion viscosity (108 dPas) lies distinctly above the processing viscosity (104 dPas) necessary for greater reshaping, the problem of heating up in mold contact is incapable of solution.